A method is provided for manufacturing flexible sheets or mats of expanded graphite material from recycled materials. The manufacturing method of the present invention may be used to manufacture flexible sheets or mats of expanded graphite material that are suitable for use in the manufacture of components in a proton exchange membrane fuel cell, such as a flow field plate or an electrode.
An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, electrodes denoted as anode and cathode surround a polymer electrolyte to form what is generally referred to as a membrane electrode assembly, or MEA. Oftentimes, the electrodes also function as the gas diffusion layer (or GDL) of the fuel cell. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell includes a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or a platinum group metal coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which joins with the protons through the proton exchange membrane and the electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit in which the electricity is utilized, and join with protons at the cathode. An air stream on the cathode side is one mechanism by which the water formed by combination of the hydrogen and oxygen is removed. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
The flow field plates may be flexible graphite sheets as described herein that are deformed into a shape (e.g., by embossing, stamping, molding, or a calender roll) that has a continuous reactant flow channel with an inlet and an outlet. The inlet is connected to a source of fuel in the case of an anode flow field plate, or a source of oxidant in the case of a cathode flow field plate. When assembled in a fuel cell stack, each flow field plate functions as a current collector.
Electrodes may be formed by providing a graphite sheet as described herein and providing the sheet with channels, which are preferably smooth-sided, and which pass between the parallel, opposed surfaces of the flexible graphite sheet and are separated by walls of compressed expandable graphite. It is the walls of the flexible graphite sheet that actually abut the ion exchange membrane, when the inventive flexible graphite sheet functions as an electrode in an electrochemical fuel cell.
The channels are formed in the flexible graphite sheet at a plurality of locations by mechanical impact. Thus, a pattern of channels is formed in the flexible graphite sheet. That pattern can be devised in order to control, optimize or maximize fluid flow through the channels, as desired. For instance, the pattern formed in the flexible graphite sheet can comprise selective placement of the channels, as described, or it can comprise variations in channel density or channel shape in order to, for instance, equalize fluid pressure along the surface of the electrode when in use, as well as for other purposes which would be apparent to the skilled artisan.
The impact force is preferably delivered using a patterned roller, suitably controlled to provide well-formed perforations in the graphite sheet. In the course of impacting the flexible graphite sheet to form channels, graphite is displaced within the sheet to disrupt and deform the parallel orientation of the expanded graphite particles. In effect the displaced graphite is being xe2x80x9cdie-moldedxe2x80x9d by the sides of adjacent protrusions and the smooth surface of the roller. This can reduce the anisotropy in the flexible graphite sheet and thus increase the electrical and thermal conductivity of the sheet in the direction transverse to the opposed surfaces. A similar effect is achieved with frusto-conical and parallel-sided peg-shaped flat-ended protrusions.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as xe2x80x9cflexible graphitexe2x80x9d). The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is as much as about 80 times or more the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
Methods of manufacturing articles from graphite particles have been proposed. For example, U.S. Pat. No. 5,882,570 to Hayward discloses a method of grinding flexible unimpregnated graphite foil to a small particle size, thermally shocking the particles to expand them, mixing the expanded graphite with a thermoset phenolic resin, injection molding the mixture to form low density blocks or other shapes, then heat treating the blocks to thermoset the material. The resulting blocks may be used as insulating material in a furnace or the like. WO 00/54953 and U.S. Pat. No. 6,217,800, both to Hayward further describe processes related to those of U.S. Pat. No. 5,882,570.
The Hayward processes are very limited in the scope of the source materials they use, and the type of end products they can produce. Hayward uses only unimpregnated graphite source materials, and his finished products are only formed by mixing the graphite powder with large proportions of resin and injection molding the mixture to form articles which are then thermoset.
Accordingly, there is a continuing need in the art for improved processes for producing flexible graphite sheets or products from various types of graphite materials, including those which are already resin impregnated, and for manufacture of more broadly useful products from those materials. Such improved processes are provided by the present invention.
As stated above, the present invention provides a method of manufacturing flexible sheets or mats of expanded graphite material. The mats manufactured by the process of the present invention are particularly useful in the manufacture of components in a PEM fuel cell, including electrodes and flow field plates.
In the production and use of flexible graphite sheets, scrap material may be generated. For example, in the production of flow field plates, a flexible graphite sheet may be shaped, impregnated with a resin, and after impregnation, cured. During this process, scrap flexible graphite sheet material may be produced before impregnation, after impregnation and before curing, and after impregnation and after curing. The scrap flexible graphite sheet material used before impregnation is described herein as regrind material or virgin regrind material. Sheet material produced after impregnation and before curing is described herein as uncured impregnated scrap (production scrap). The material produced after impregnation and after curing is described herein as cured regrind (regrind scrap). Furthermore, the present invention may use, as a source material, sheet material produced from recycled used material such as recycled fuel cell flow field plates.
Using the methods of the present invention, one can regrind the above sheet material and recycle the same into flexible graphite mats that may be further processed for use, e.g., as a material which can be formed into a component in a fuel cell.
The method of the present invention is advantageous because it provides a beneficial re-use of flexible graphic sheet material such as the uncured impregnated scrap and cured impregnated scrap used in the production of, for example, flow field plates. The present invention provides an advantageous use for such material and decreases disposal costs.
Specifically, one embodiment of the present invention is a method of manufacturing flexible sheets of expanded graphite material from recycled materials, comprising providing source materials in the form of flexible sheets of expanded graphite; comminuting the source materials into particles; re-expanding the particles; and preparing a mat from the re-expanded particles.
Another embodiment of the present invention is a process of manufacturing a graphite material comprising grinding a cured resin impregnated graphite material into particles; removing at least part of the resin from the particles; and expanding the resin removed particles.
It is an object of the present invention to provide a method for preparing graphite mats that can be manufactured from recycled materials.
Yet another object of the present invention is to provide material suitable for the construction of a component of a fuel cell manufactured using expanded graphite material from recycled materials.
Still another object of the present invention is to provide a method for manufacturing flexible sheets of graphite material from, as a source material, unimpregnated graphite sheet material, uncured resin impregnated graphite sheet material, and cured resin impregnated graphite sheet material.
Another object of the present invention is to provide a method of manufacturing flexible sheets of expanded graphite material from, as a source material, used, recycled, graphite material such as flow field plates and electrodes.
Another object of the present invention is to provide a process for manufacturing graphite material comprising: recycling resin impregnated graphite material and removing the resin from the recycled graphite material. The process of this embodiment includes removing at least part of the resin from the source material and re-expanding the resin removed particles.
Other and further objects, features, and advantages would be readily apparent to those skilled in the art, upon a reading of the following disclosure when taken in conjunction with the accompanying drawings.